The subject matter of this invention relates generally to inverter type power conversion systems and more particularly to inverter systems which are operable in two distinct modes, namely a time ratio controlled mode (pulse-width modulation) and a square wave mode. The present invention is especially applicable to those situations where the DC power to the inverter is monitored but not directly controlled by the inverter control system.
Generally, alternating electrical current loads are best suited for alternating electrical current power sources and direct current electrical loads are best suited for direct current electrical power sources. For instance, alternating current induction motors in the past have been driven by alternating current sources of electrical power and direct current motors have been driven by direct current sources of electrical power. Alternating current electrical motors are usually relatively more efficient than direct current electrical motors in converting a given amount of electrical power into rotational energy, whereas direct current electrical motors are easily adaptable for speed control while alternating current electrical motors such as induction motors are not. In any motor application where speed control is not important the alternating electrical current motor is usually chosen. This is because of the previously mentioned efficiency and the additional fact that alternating electrical current is the type that is most readily available because of its superior transmission and distribution characteristics. On the other hand, if speed control is important, the available alternating current's input power is converted by way of a diode bridge-filter-voltage regulator apparatus to direct current which is then utilized to empower the easily speed-controlled DC motor. Until the early 1970's the designer's choice of motor system was usually based on the foregoing criteria. However, with the advent of the so-called energy crisis the efficiency of the AC motor relative to that of the DC motor became exceedingly more important. Consequently, in situations where speed-controlled relatively inefficient DC motors would have normally been chosen, designers and users began to attempt to find ways to adapt speed control techniques to the efficient alternating current motors, especially the simple induction motor. Inverter technology which generally converts DC electrical power into AC electrical power, the opposite of rectifier technology became the cornerstone of the aforementioned attempt. The use of a control system to generate variable frequency AC square waves, when combined with inverter technology, provided a logical means for implementing the solution. Furthermore, the use of pulse-width-modulation techniques in conjunction with the square wave generation extended the frequency range over which speed control of an AC motor could be implemented. It is a known characteristic of AC motors that the machine performs at a high level of efficiency if the phase voltage across the windings thereof varies proportionally to the frequency, that is the higher the frequency the higher the phase voltage. In the past, operating in the pure square wave mode of operation as the controller increased the square wave frequency a signal was provided to a device controlling the DC input power to cause the voltage thereof to increase proportionally. On the other hand, in the pulse-width-modulated mode of operation, as the overlying frequency was varied, pulses were generated to be mixed with the square wave signal in order to create notches therein which had the effect of reducing the overall voltage for relatively lower frequencies and proportionally increasing the overall voltage for relatively higher frequencies, all within the pulse-width-modulated mode of operation. As the frequency required grew larger in the pulse-width-modulated mode of operation, the notches and correspondingly the pulses which produced those notches, became correspondingly smaller so that the overall voltage increased proportionally. Eventually, near the transition frequency between the pulse-width-modulation mode of operation and the pure square wave mode of operation, the notch creating pulses became so narrow in time that the power switching devices in the inverter could not properly duplicate them. This is due to such phenomena, for example, as snubber interreaction and commutation recovery. A simple solution in the past was to simply stop generating pulses in the pulse-width modulation mode of operation when the pulses became too narrow to be appropriately handled by the inverter power circuitry and merely to jump to the higher frequency pure square wave mode of operation with an attendant step function discontinuity in motor voltage. The discontinuity had the effect of increasing the average voltage applied to the load in a step function. This sudden increase in voltage had a tendency to cause problems in the AC motor, which problems may be associated with oscillation among other undesirable characteristics. One prior art solution was to correspondingly modify the already controlled DC input voltage to the inverter so that the effect of the discontinuity could be cancelled out. Reference is made to U.S. Pat. No. 3,870,945, issued Mar. 11, 1975 to N. P. Pedersen et al in which the foregoing is set forth.
As the state of the art in inverter technology has increased over the years, the demand for simplicity and a further reduction in the expense of operation has increased correspondingly. For example, it would be very desirous for relatively uncomplicated situations not to have to increase the voltage proportionally with an increase in frequency in the pure square wave mode of operation. Said in another way it would be very desirable to operate the motor in the square wave mode of operation at a constant voltage. If this is allowed to happen, then no control is necessary for the DC input voltage, it being remembered that voltage control in the pulse-width-modulation mode of operation is provided by the "chopping" technique described previously where pulses are utilized to create notches in the basic square wave. The elimination of voltage control devices and equipment for the input to the inverters is very desirable as that obviously leads to a less expensive inverter system. It does mean, however, that except for variations in input voltage due to line phenomena, etc., for example, that the inverter system is operated at a constant DC input voltage over its entire frequency range of operation. It is, however, desirable to monitor the input DC voltage as a variation of the DC voltage due to line transients and other phenomena causing a slight short term raising or lowering of the DC voltage which can have a negative effect on the motor controlled by the inverter. To do this it would be desirable to monitor the DC voltage and provide a feed forward signal to the control circuit for the inverters so that slight changes in DC voltages can be accommodated for in a feed forward fashion, for example. This very desirable feature has a drawback associated with the previously described transition region between the pulse-width-modulation mode of operation and the constant voltage variable frequency mode of operation. The undesirable feature is associated with the fact that eliminating pulses as is done in the prior art and as is necessary in the present state of the art and then quickly moving into the constant voltage pure square wave mode of operation results in the previously described sudden increase in motor voltage when the notches are first ignored. When the motor voltage is increased the motor current decreases. However, the DC source is never a perfect voltage source and consequently decreases in the motor current will cause a correlated rise in the input voltage to the inverter. This rise in voltage is monitored by the control system. The response to the slight rise in DC input voltages is for the control system to request a slightly wider chopping notch. If the inverter system is operating near the transition frequency, the notch will become wide enough for the inverter switches to recognize and handle it. This will result in a sudden decrease in motor voltage and a corresponding increase in motor current causing the DC input voltage to droop. The control responds to this to narrow the notches, starting the cycle over again. Thus it can be seen that with a feed forward voltage sensor for the DC input voltage to the inverter where pulses below a certain minimum time value are ignored very undesirable oscillation can occur near the transition region between the pulse-width-modulation mode of operation and the pure square wave mode of operation. It would be desirable therefore if apparatus could be provided which would alleviate the foregoing problem.